For harmonic analysis it is not practically possible to model a large
transmission network with its neighboring interconnected systems in de
tail due to computer time and memory limitations. Furthermore, a large
number of outage contingencies has to be analyzed to compute harmonic
impedance boundaries for filter designs. Current methods do not addre
ss modeling simplification errors and also accuracy of harmonic impeda
nce boundaries, and depend mostly on trial and error or individual exp
erience approaches. A methodology has been presented to model transmis
sion network more accurately and efficiently by using adjoint network
sensitivity analysis and bilinear theorem. A new method is also presen
ted that reduces the number of outage contingencies to be analyzed by
a large factor, that considerably reduces the engineering and computer
time, and increases dependability.